Method and arrangement for fixing toner images applied to a web-shaped recording medium with high quality consistency

ABSTRACT

In a fixing station operating according to the principle of cold fixing in an electrophotographic printer means, a control of the influencing time of the solvent vapor on the recording medium ensues in addition to the control of the vapor concentration. To this end, the recording medium is immersed to different depths into a container containing a solvent vapor, being immersed thereinto via a deflection roller dependent on the operating conditions.

The invention is directed to a method for fixing toner images applied toa web-shaped recording medium and to an arrangement for theimplementation of the method according to the preamble of patent claims1 and 2.

German Patent 28 38 864 discloses an apparatus of the species initiallycited. For fixing toner powder applied to a recording medium, forexample a paper web, in electrostatic copier or recording devices, therecording medium is transported through a fixing station in which thetoner is joined fast to the recording medium. The fixing stationcontains a container through which the recording medium is conductedloop-shaped. Solvent vapor is generated at the underside of thecontainer. A deflection means around which the recording medium isconducted is arranged in the region of the container enriched withsolvent vapor. The recording medium is thereby exposed to the solventvapor. After fixing, the recording medium departs the fixing region andis in turn conducted out of the container via an upper deflectingroller. The fixing region is limited in that cooling coils are arrangedin the container wall in the upper region of the container. A cooledregion in which solvent vapor emerging from the fixing region condensesthereby arises in the container. The lower deflection means can beremoved from the container for introducing the recording medium.

Such non-mechanical copier or printer devices operating on thecold-fixing method are based on the principle that toner can bedissolved or, respectively, volatilized under the influence of solventvapor and, thus, can penetrate into the surface of the recording medium.The degree of softening or, respectively, volatilization of the tonermaterial depends critically on the concentration of the solvent vaporand on its influencing time.

The recording medium is thereby drawn through the vapor region of thefixing means with constant speed in the form of a loop. The reactiontime between toner and solvent thus becomes a constant quantity insystem-conditioned fashion.

In order to achieve the goal of a uniform fixing quality, theconcentration of the solvent or, respectively, fixant vapor must beexactly held to a corresponding value.

To this end, German Patent 31 11 970 discloses a circuit arrangement ina means for monitoring the concentration of an air-vapor mixture in atoner image fixing station working with solvent vapor.

Fluctuations in the fixing strength, as may nonetheless still occur, canno longer be eliminated even by an improvement of the control outlay forthe vapor concentration. The reason lies therein that the solvent(fixing means) is replenished in liquid form when the ratedconcentration is fallen below and must then be evaporated. Theevaporation speed therefore led to a thermically conditioned dead timefor the control system. A further dead time arises given an excess ofvapor, i.e. an excessively high concentration of the solvent vapor. Thisis dismantled essentially only as a consequence of condensation at acold trap arranged in the container for the solvent vapor and cannot beinfluenced in control-oriented terms.

The fixing strength decreases given a decrease in the vaporconcentration. Given an increasing concentration, the toner dissolves toa more intense degree. The time span that elapses from the emergence ofthe recording medium from the fixing station up to deposit on a fanfolddeck is available for the subsequent drying process.

When printed paper is deposited in the form of a fanfold deck without anadequate quantity of solvent vapor having dried, this leads thereto thatthe tonered (inked) regions of the paper web stick to one another. Thisparticularly occurs when the deck part stacked thereabove compresses theregion that lies lower. The residue of solvent that is still effectivein this case allows the toner adhering to the paper web to harden onlygradually. The more or less pasty consistency of the toner harbors therisk that the toner layers touching one another will fuse to oneanother. This must be avoided under all circumstances since fragments ofthe informational content of the recording medium would otherwise bedestroyed when the stacked sheets that have glued together are foldedopen later.

In known fixing stations, the rated concentration is selected somewhatabove the optimum vapor concentration of solvent in order to reliablysuppress an inadequate (weak) fixing under all operating conditions. Thedisadvantage that printed pages will stick together at the operatingtemperatures close to the upper limit of the device specification oreven given intermittent printing operation with brief printing andpausing phases is thereby consciously accepted.

It is an object of the invention to fashion an apparatus and a method ofthe species initially cited such that an optimum fixing quality isachieved under all operating conditions, regardless of fluctuations ofthe solvent concentration in the fixing station.

In a method of the species initially cited, this object is achieved inthat a control of the influencing time of the solvent vapor on therecording medium is carried out by adjusting the passage distance of therecording medium through the solvent vapor container in order to achievea uniform fixing quality given fluctuating operating conditions (inkingdegree, air pressure, paper temperature).

As a consequence of the inertia in producing the solvent vapor, theconcentration of solvent vapor changes relatively slowly and does notalways adapt to the changing conditions with adequate rapidity. Thisproblem is intensified given high printing speeds as are typical fornon-mechanical fast printers that operate according to the principle ofelectrophotography. In that the influencing time of the solvent vapor onthe recording medium is controlled via the passage distance, a fast andunproblematical possibility of being able to quickly compensatefluctuations in the solvent vapor concentration derives.

In an advantageous arrangement for the implementation of the method ofthe invention, the guide means are arranged displaceable in positioninside the container via an electromotive means for adjusting theinfluencing time of the solvent vapor on the recording medium such thata modification of the position of the guide means effects a modificationof the passage distance of the recording medium through the container.

The container for the acceptance of the solvent vapor that is open atone side in a known fashion comprises deflection mechanisms for therecording medium at its upper side and in its floor region. The lowerdeflection mechanism is thereby arranged movable parallel to the longsides of the container inside the same container, being movable via theelectromotive means.

An embodiment of the invention is shown in the drawings and shall be setforth in greater detail below by way of example. Shown are:

FIG. 1 a schematic block illustration of an arrangement for fixing tonerimages in a printer means operating according to the principle ofelectrophotography;

FIG. 2 a schematic illustration of a servo drive employed in thearrangement;

FIGS. 3 through 5 various acceleration and retardation curves givendifferent lanes of the deflection means according to FIG. 1.

As set forth in greater detail in German Patent 28 38 864, a recordingmedium AD inked with toner is guided via deflection rollers U1, U2, U3through a container BH containing a solvent vapor L in a printer meansoperating on the principle of electrophotography (not shown in detailhere) and the toner image is thus fixed on the recording medium AT. Thecontainer BH is executed deep and is open only at its upper side OS. Thefixing of the toner image on the recording medium AT ensues in its lowerregion US. The solvent vapor has such a concentration in this region USthat the toner image is melted on the recording medium AT and canpenetrate into the recording medium. This region is called the fixingregion. The solvent vapor is supplied to the fixing region US. In theexemplary embodiment, this ensues in that solvent is introduced into thecontainer BH through a pipe RO. A heating heating means HE, for examplea heating coil, can be arranged at the underside SS of the container BH,the solvent being heated by this heating means HE and being convertedinto its vapor form. A vapor sensor DS that can be constructed in aknown way must be provided for measuring the vapor concentration in thefixing region US.

In order to enable the fixing of the toner image on the recording mediumAT, the latter must be conducted through the fixing region US togetherwith the toner images. To that end, a lower deflection means--a rollerU3 in the exemplary embodiment --is provided in the fixing region of thecontainer BH. The recording medium AT is conducted around this lowerfixing means U3 such that the tonered side faces away from the lowerdeflection means. The recording medium AT is thereby conducted throughthe container loop-like. It comes from the open, upper side OS of thecontainer BH, proceeds to the lower deflection means U3 and is conductedback to the upper side of the container.

In order to prevent an escape of the solvent vapor from the fixingregion US, cooling coils KL are arranged in the container wall above thefixing region US. The solvent vapor that emerges from the fixing regionUS arrives into a cooled zone where it condenses. In order to promotethis process, it is expedient to employ a solvent that is specificallyheavier than air in its vapor form. It must be pointed out that thevapor concentration does not suddenly decrease upon transition from thefixing region US to the cooled region; rather, it decreases gradually.However, the vapor concentration is so high only in the fixing region USthat a fixing of the toner on the recording medium AT is possible.

With the assistance of an electromotive drive means M, the lowerdeflection roller U3 can be displaced in the container along thecontainer walls via guide means S that, for example, can be a componentpart of a cable drive. The actual fixing distance FS in the fixingregion US can thus be adjusted.

Given an assumed, constant speed of passage of the recording medium ATthrough the container BH, the influencing time of the solvent vapor L onthe recording medium is varied by varying the fixing distance FS.Different immersion depths of the deflection roller U3 thus result indifferent influencing times of the solvent vapor L.

Two control circuits are then essentially provided for controlling thefixing quality. A first control circuit controls the vapor concentrationin the container BH dependent on the operating conditions. To this end,a sensor for the air pressure SL and a sensor for paper temperature SPare provided, the output signals thereof being acquired by amicroprocessor-controlled arithmetic unit RS that is constructed in astandard way. The microprocessor-controlled arithmetic unit RS alsoadditionally acquires an adjustable, manual correction value MK that,for example, can be input via a control keyboard (not shown here). Inaddition, the arithmetic unit RS is also in communication with theactual apparatus control GS via which, for example, the printer status(print mode, idle mode, etc.) is communicated.

The arithmetic unit calculates the optimum solvent concentration in thecontainer BH for a specific recording medium from all of the valuesacquired in this fashion. This calculated value serves as rated valuefor a further difference calculator DF that is likewise constructed inthe standard way and that acquires, first, the rated value calculated bythe arithmetic unit RS and, second, the actual value supplied by a vaporsensor DS that is converted into a correspondingly adapted signalsequence via a converter W. By forming the difference between the ratedvalue and the actual value, this difference calculator DF calculates theconveyed quantity of solvent required for achieving the optimumconcentration of solvent vapor and controls the delivery of thisconveyed quantity of solvent via a shut-off valve AS.

The control of the vapor concentration in the ways set forth isnotoriously known and, for example, is disclosed in greater detail inGerman Patent 31 11 970.

The control system for the vapor concentration is then supplemented suchthat the formation of the difference between rated and actualconcentration is evaluated not only for the replenishment quantity ofthe solvent but is also additionally evaluated for the correction of thenew fixing distance FS. Dependent on under-concentration orover-concentration, an adjustment means conducts the lower deflectionroller U3 into the position that is pre-programmed therefor. To thisend, the deviation of the control circuit calculated by forming thedifference between rated and actual value of the vapor concentration issupplied to a further control circuit for the fixing distance FS. Thisfurther control circuit contains a microprocessor-controlled arithmeticunit RF for calculating the fixing distance FS whose function islikewise fundamentally monitored by the apparatus control GS and that isconstructed in the standard way. From the deviation, the arithmetic unitRF for the fixing distance calculates the correction of the influencingtime of the solvent vapor on the recording medium required in order tomaintain a constant quality and, thus, calculates the requiredcorrection of the fixing distance FS.

In the simplest case according to FIG. 2, the required positioning ofthe lower deflection roller U3 is carried out by a drive motor M that isconnected to the output of a power operational amplifier OV. The controlvoltage SV applied to the operational amplifier OV prescribes the ratedvalue and is determined by the arithmetic unit for the fixing distanceRF via appropriate converters. A path-generating potentiometer WGmechanically coupled to the motor shaft supplies the actual value to theoperational amplifier OV. Given an adequately great difference involtage, the operational amplifier follows the drive motors M up. Ananalog control ensues in the illustrated exemplary embodiment. As setforth later, however, it is also possible to employ stepping motors thatare digitally driven instead of the servo motors.

The additional control circuit for the fixing distance always modifiesthe length of the paper loop in the vapor region to the ideal operatingpoint. Critical printing mode, for example, briefly intermittentprinting mode, high ambient temperatures, operation at high altitude(more than 1500 m above mean sea level) or the employment of specialpapers can be covered in this way with high quality consistency.

A starting and braking control AB inserted into the control circuit thatcollaborates with a corresponding digital-to-analog converter DA isintended to secure a soft run-up and stop of the drive motor M for thelower deflection roller U3. The risk that the recording medium AT willrip off due to spontaneous variation of the loop size because a slack inthe recording medium or a starting jerk briefly appears is thusprevented. Under certain circumstances, such a sudden operation wouldresult in a distorted imaging of the line to be transferred.

Problems of this type are known from EP-A2-01 15 866 under thedesignation "smudged printing".

The starting and braking control AB can be a filter acting as a low-passfilter, so that sudden changes are forwarded from the output of thedigital-to-analog converter in damped fashion. a soft run-up of theservo motor thereby derives. The soft braking event when the targetposition is approached automatically derives by decrease of the voltagedifference (control voltage/path-generator voltage) at the inputs of theoperational amplifier.

Given employment of a stepping motor as electromotive means M comprisingan allocated, digital position transmitter WG according to FIG. 1, it isexpedient to control the entire travel distance FW of the deflectionroller U3 in program-controlled fashion via a microprocessor CPU. Tothat end, the optimum acceleration curve BK or, respectively,retardation curve VK (FIG. 3-FIG. 5) is programmed in a memory ROM. Thememory cells F1-F8 contain finely graduated digital information thatdetermine the RPMs or, respectively, the stepping frequencies SF, forexample F1=1, F8=8.

Upon acceleration BS, the memory cells F1 . . . F8 are successivelyinterrogated with increasing stepping frequencies 1-8 up to the maximumvalue. This occurs in the inverse sequence during the retardation VS(see FIG. 3). When the travel distance FW is shorter than twice theaccelerating distance BS, the incrementation of the memory cells F1-F8is interrupted at half the travel distance F4 and is again reversed. Thehighest possible stepping frequency SF is then not reached (see FIG. 4).The maximum stepping frequency SF=8, memory cell F8, is thus onlyreached when the travel distance FW is at least as long as twice theaccelerating distance BS. When the travel distance TW is longer thantwice the accelerating distance BS, the maximum stepping frequency(memory cell F8) is retained up to the beginning of the retardationcurve VS of equal length (distance of constant speed KS).

The travel distance FW of the deflecting roller U3 that is divided intointervals ΔFW, 1-16 of equal length is thus divided into two respectivesub-distances having an interval number ΔFW of the same size. The firstdistance is the accelerating distance BS; the second distance VS retardsthe speed again. When, however, the interval number ΔFW, 1-16 is greaterthan twice the memory cell number F1-F8 or when 1 remains as theremainder of the division, the deflection roller U3 travels at constantspeed in this section KS between acceleration curve BS and retardationcurve VS (see FIG. 5). STOP thereby references the resting condition ofthe deflection roller before and after the motion.

I claim:
 1. An arrangement for fixing toner images applied to aweb-shaped recording medium with a solvent vapor, comprising:a containercontaining a solvent vapor in a fixing region; a guide means for guidinga recording medium into said fixing region in said container, said guidemeans being arranged displaceable in position within the container; anelectromotive means for varying the position of said guide means in saidcontainer to effect a modification of a passage distance of therecording medium through the fixing region of the container; at leastone sensor for sensing an operating condition of the arrangement forfixing; and a control circuit for controlling said electromotive meansto modify the passage distance of the recording medium through thefixing region to compensate for variations in the operating conditionssensed by said sensors.
 2. An arrangement according to claim 1, whereinthe container is open at one side being an upper side, said containerincluding a floor region opposite said upper side and long sidesextending therebetween; andwherein the guide means for guiding therecording medium includes a lower guide arranged movable in thecontainer via the electromotive means, being arranged movable parallelto the long sides of said container.
 3. An arrangement according toclaim 1, wherein said control circuit is a second control circuit, andfurther comprising a first control circuit that controls a solventconcentration by supplying solvent.
 4. An arrangement according to claim3, wherein the first control circuit forms a deviation signal by formingthe difference between an actual signal corresponding to a currentsolvent concentration and a rated signal calculated taking the operatingconditions into consideration, the deviation signal is supplied to thesecond control circuit, the second control circuit comprises acalculating means that calculates the required modification of thefixing distance and adjusts said electromotive means dependent thereon.5. An arrangement according to claim 4, further comprising:a positionaltransmitter being a distance-generating potentiometer coupled to theelectromotive means; and an operational amplifier that adjusts theelectromotive means dependent on a control voltage derived from thedeviation signal and dependent on an output signal of the positionaltransmitter.
 6. An arrangement according to claim 4, wherein theelectromotive means comprises a stepping motor having an allocated,digital position transmitter.
 7. An arrangement according to claim 4,further comprising:a program-controlled arrangement that comprises amemory for the acceptance of acceleration values and of retardationvalues for the movement of the guide means.
 8. A method as claimed inclaim 1, wherein said step of automatically adjustingincludes:determining a deviation of an actual vapor concentration fromthe rated vapor concentration in said fixing region; determining acorrection of a fixing distance of the recording medium through thefixing region depending upon the deviation of the actual vaporconcentration from the rated vapor concentration; and controlling anelectromotive device to vary said fixing distance of said recordingmedium in accordance with said correction determined in the precedingstep.